1. Field of the Invention
The present invention generally relates to an apparatus for controlling current supply to a heater in an air-fuel ratio sensor in the exhaust passage of an internal combustion engine. More particularly, the present invention relates to a current controlling apparatus including a plurality of air-fuel sensors.
2. Description of the Related Art
An air-fuel ratio sensor is located in the exhaust passage of an external combustion engine for detecting the concentration of oxygen in the exhaust gas. The air-fuel ratio of the air-fuel mixture is computed based on the detected oxygen concentration. Typically, the computed air-fuel ratio is then compared with a predetermined target air-fuel ratio (usually a theoretical optimum air-fuel ratio). The amount of fuel in the mixture is feedback controlled such that the detected ratio becomes equal to the predetermined target ratio.
Japanese Unexamined Patent Publication No. 63-176641 discloses "an apparatus for controlling the air-fuel ratio in an internal combustion engine". This apparatus includes two O.sub.2 sensors (air-fuel ratio sensor) located at the upstream and downstream sides of a three way catalytic converter. The air-fuel ratio of the engine is controlled based on the signals from the sensors. Employing two air-fuel sensors allows the actual air-fuel ratio to be accurately controlled to match a target air-fuel ratio.
Each air-fuel sensor in the above apparatus includes an element and a heater for warming the element. The element is activated when its temperature is equal to or higher than a predetermined temperature. The activated element allows the sensor to operate. The heater warms the sensor such that the temperature of the element is, for example, 350.degree. C. to 400.degree. C. thereby activating the element.
The recent trend of environmental protection has resulted in a stricter regulation of exhaust gas. For example, it is desired that the amount of hydrocarbon (HC) exhausted from an engine when the engine is started be minimized. Specifically, it is desired that the element in an air-fuel ratio sensor be activated at an early stage after starting of the engine. This would allow the air-fuel ratio to be adequately controlled in an early stage. One of the ways to activate the element in an early stage is to increase the heating value of the heater by lowering the value of resistance of the heater.
However, in an engine having a plurality of air-fuel ratio sensors, increasing the heat value of the heaters increases the power consumption of the heaters. FIG. 9 is a graph showing changes of current value supplied to a heater. As shown in the graph, a current having a relatively great value is supplied to the heater at the beginning of the current supply to the heater. This current is referred to as a rush current. Then, the value of the current supplied to the heater is gradually reduced until the value reaches a predetermined constant level.
If two or more heaters are parallel-connected to an electrical power source, current is simultaneously supplied to the heaters when the engine is started. In this case, current in the power source, or in the battery, is equal to or greater than the sum of the rush currents supplied to the heaters. Power consumption is thus extremely high. Therefore, starting the engine increases the load on the battery and the load on an electrical circuit for supplying the electricity of the battery (hereinafter referred to as a power source system). In particular, the increase in the power consumption drastically lowers the voltage of the battery when starting the engine or when the battery charge is relatively low.
Typical heaters are controlled by periodic ON-OFF signals as shown in FIG. 10, or duty signals, when the element is activated. In this case, currents I4 and I5 are supplied to the parallel-connected heaters, respectively.
If the two duty signals are synchronized with each other, the sum of the currents I4, I5 fluctuate periodically. Specifically, between a time T1 and a time T2, the value of each current is 2IA! and the sum is 4IA!, while the value of each current and the sum is 0A! between the time T2 and a time T3. In general, the chemical reaction in the battery electrolyte becomes too slow when the discharge current of the battery is great. This lowers the voltage of the battery. Fluctuations of the sum of the currents I4, I5 as described above therefore fluctuate the battery voltage. The battery voltage fluctuation causes problems in controlling the parts of the engine. For example, fluctuations of voltage applied to injectors deteriorates accuracy of fuel injection control. This results in unstable engine idling.